Dynamic verify based on threshold voltage distribution

ABSTRACT

After erasing a plurality of non-volatile storage elements, a soft programming process is performed to tighten the erase threshold distribution for the non-volatile storage elements. During the soft programming process, the system identifies the number of programming pulses needed for a first set of the non-volatile storage elements to complete the soft programming and the number of programming pulses needed for the all but a last set of non-volatile storage elements to complete soft programming. These two numbers are used to characterize the threshold distribution of the non-volatile storage elements. This characterization of the threshold distribution and the program pulse step size are used to limit the number of verify pulses used during subsequent programming.

This application claims the benefit of provisional application No.60/890,832, “Diagonal Sequence Write,” by Nima Mokhlesi, filed on Feb.20, 2007, incorporated herein by reference in its entirety.

CROSS-REFERENCE TO RELATED APPLICATIONS

The following applications are cross-referenced and incorporated byreference herein in their entirety: U.S. patent application Ser. No.______ [Attorney Docket No. SAND-01237US0], titled “Multiple Pass WriteSequence For Non-Volatile Storage” Inventor Nima Mokhlesi, filed thesame day as the present application; U.S. patent application Ser. No.______ [Attorney Docket No. SAND-01237US2], titled “Non-Volatile StorageApparatus With Multiple Pass Write Sequence,” Inventor Nima Mokhlesi,filed the same day as the present application; U.S. patent applicationSer. No. ______ [Attorney Docket No. SAND-01239US0], titled “VariableInitial Program Voltage Magnitude For Non-Volatile Storage,” InventorNima Mokhlesi, filed the same day as the present application; U.S.patent application Ser. No. ______ [Attorney Docket No. SAND-01239US1],titled “Non-Volatile Storage Apparatus With Variable Initial ProgramVoltage Magnitude,” Inventor Nima Mokhlesi, filed the same day as thepresent application; and U.S. patent application Ser. No. ______[Attorney Docket No. SAND-01240US1], titled “Non-Volatile StorageApparatus With Dynamic Verify Based on Threshold Voltage Distribution,”Inventor Nima Mokhlesi, filed the same day as the present application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to technology for non-volatile storage.

2. Description of the Related Art

Semiconductor memory has become more popular for use in variouselectronic devices. For example, non-volatile semiconductor memory isused in cellular telephones, digital cameras, personal digitalassistants, mobile computing devices, non-mobile computing devices andother devices. Electrical Erasable Programmable Read Only Memory(EEPROM) and flash memory are among the most popular non-volatilesemiconductor memories.

Both EEPROM and flash memory utilize a floating gate that is positionedabove and insulated from a channel region in a semiconductor substrate.The floating gate and channel regions are positioned between the sourceand drain regions. A control gate is provided over and insulated fromthe floating gate. The threshold voltage of the transistor is controlledby the amount of charge that is retained on the floating gate. That is,the minimum amount of voltage that must be applied to the control gatebefore the transistor is turned on to permit conduction between itssource and drain is controlled by the level of charge on the floatinggate.

When programming an EEPROM or flash memory device, such as a NAND flashmemory device, typically a program voltage is applied to the controlgate and the bit line is grounded. Electrons from the channel areinjected into the floating gate. When electrons accumulate in thefloating gate, the floating gate becomes negatively charged and thethreshold voltage of the memory cell is raised so that the memory cellis in a programmed state. More information about programming can befound in U.S. Pat. No. 6,859,397, titled “Source Side Self BoostingTechnique for Non-Volatile Memory;” U.S. Pat. No. 6,917,542, titled“Detecting Over Programmed Memory;” and U.S. Pat. No. 6,888,758, titled“Programming Non-Volatile Memory,” all three cited patents areincorporated herein by reference in their entirety.

In many cases, the program voltage is applied to the control gate as aseries of pulses (referred to as programming pulses), with the magnitudeof the pulses increasing at each pulse. Between programming pulses, aset of one or more verify operations are performed to determine whetherthe memory cell(s) being programmed have reached their target level. Ifa memory cell has reached its target level, programming stops for thatmemory cell. If a memory cell has not reached its target level,programming will continue for that memory cell.

Some EEPROM and flash memory devices have a floating gate that is usedto store two ranges of charges and, therefore, the memory cell can beprogrammed/erased between two states (an erased state and a programmedstate). Such a flash memory device is sometimes referred to as a binarymemory device.

A multi-state memory device stores multiple bits of data per memory cellby identifying multiple distinct valid threshold voltage distributions(or data states) separated by forbidden ranges. Each distinct thresholdvoltage distribution corresponds to a predetermined value for the set ofdata bits encoded in the memory device. For example, a memory cell thatstores two bits of data uses four valid threshold voltage distributions.A memory cell that stores three bits of data uses eight valid thresholdvoltage distributions.

As the number of bits of data per memory cell (and, therefore, thenumber of valid threshold voltage distributions) are increased, the datacapacity of a memory device increases. However, the time needed forprogramming also increases. For example, the greater the number of validthreshold voltage distributions, the greater the number of verifyoperations needed between program pulses. Users typically do not want towait for their electronic devices to store data. For example, users ofdigital cameras do not want delays between taking photographs.

As memory devices store more bits of data per memory cell, the need forreasonable program speeds has increased.

SUMMARY OF THE INVENTION

A process is disclosed that limits the amount of verify operationsbetween program pulses in order to improve programming speed.

One embodiment includes performing a first programming process for aplurality of non-volatile storage elements and performing a secondprogramming process for the non-volatile storage elements subsequent tothe first programming process. The second programming process includes aset of verify operations that are at least partially based on the firstprogramming process.

One embodiment includes performing a first programming process for aplurality of non-volatile storage elements. The first programmingprocess includes applying programming pulses to the non-volatile storageelements. The method further comprises identifying a first program pulseneeded to achieve a first result during the first programming process,identifying a second program pulse needed to achieve a second result forthe first programming process, and performing a second programmingprocess for the non-volatile storage elements subsequent to the firstprogramming process. The second programming process uses verify pulsesbetween program pulses. The verify pulses are limited based on the firstprogram pulse and the second program pulse.

One embodiment includes erasing a plurality of non-volatile storageelements, soft programming the plurality of non-volatile storageelements in association with the erasing, identifying a first programpulse associated with a first set of one or more of the non-volatilestorage elements completing the soft programming, identifying a secondprogram pulse associated with a second set of one or more of thenon-volatile storage elements completing the soft programming, andprogramming the non-volatile storage elements. The programming processincludes performing verify operations based on the first number ofprogramming pulses and the second number of programming pulses.

One example implementation includes a set of non-volatile storageelements and one or more managing circuits in communication with the setof non-volatile storage elements. The one or more managing circuitsperform the processes described above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of a NAND string.

FIG. 2 is an equivalent circuit diagram of the NAND string.

FIG. 3 is a block diagram of a non-volatile memory system.

FIG. 4 is a block diagram depicting one embodiment of a memory array.

FIG. 5 is a block diagram depicting one embodiment of a sense block.

FIG. 6 depicts an example set of threshold voltage distributions.

FIG. 7 depicts an example set of threshold voltage distributions.

FIG. 8 depicts an example coding of data into a set of data statesassociated with threshold voltage distributions.

FIG. 9 depicts an example coding of data into a set of data statesassociated with threshold voltage distributions.

FIG. 10 is a flow chart describing one embodiment of a process foroperating non-volatile storage.

FIGS. 11A-D depict a multi-pass programming process.

FIG. 12 is a flow chart describing one embodiment of a process forprogramming non-volatile storage.

FIG. 12A depicts programming pulses and verify pulses applied tocontrols gates of selected memory cells.

FIG. 13 is a chart describing one embodiment of a sequence forperforming the four pass programming process on multiple word lines.

FIG. 14 is a flow chart describing one embodiment of a process foroperating non-volatile storage.

FIG. 15 is a flow chart describing one embodiment of a process forprogramming non-volatile storage.

FIG. 16 is a flow chart describing one embodiment of a process foroperating non-volatile storage.

FIG. 17 is a flow chart describing one embodiment of a process foroperating non-volatile storage.

FIG. 18 is a flow chart describing one embodiment of a process forverifying.

FIG. 19 is a flow chart describing one embodiment of a process forreading data.

DETAILED DESCRIPTION

One example of a flash memory system uses the NAND structure, whichincludes arranging multiple transistors in series, sandwiched betweentwo select gates. The transistors in series and the select gates arereferred to as a NAND string. FIG. 1 is a top view showing one NANDstring. FIG. 2 is an equivalent circuit thereof. The NAND stringdepicted in FIGS. 1 and 2 includes four transistors 100, 102, 104 and106 in series and sandwiched between a first (or drain side) select gate120 and a second (or source side) select gate 122. Select gate 120connects the NAND string to a bit line via bit line contact 126. Selectgate 122 connects the NAND string to source line 128. Select gate 120 iscontrolled by applying the appropriate voltages to select line SGD.Select gate 122 is controlled by applying the appropriate voltages toselect line SGS. Each of the transistors 100, 102, 104 and 106 has acontrol gate and a floating gate. For example, transistor 100 hascontrol gate 100CG and floating gate 100FG. Transistor 102 includescontrol gate 102CG and a floating gate 102FG. Transistor 104 includescontrol gate 104CG and floating gate 104FG. Transistor 106 includes acontrol gate 106CG and a floating gate 106FG. Control gate 100CG isconnected to word line WL3, control gate 102CG is connected to word lineWL2, control gate 104CG is connected to word line WL1, and control gate106CG is connected to word line WL0.

Note that although FIGS. 1 and 2 show four memory cells in the NANDstring, the use of four transistors is only provided as an example. ANAND string can have fewer than four memory cells or more than fourmemory cells. For example, some NAND strings will include eight memorycells, 16 memory cells, 32 memory cells, 64 memory cells, 128 memorycells, etc. The discussion herein is not limited to any particularnumber of memory cells in a NAND string.

A typical architecture for a flash memory system using a NAND structurewill include several NAND strings. Each NAND string is connected to thesource line by its source select gate controlled by select line SGS andconnected to its associated bit line by its drain select gate controlledby select line SGD. Each bit line and the respective NAND string(s) thatare connected to that bit line via a bit line contact comprise thecolumns of the array of memory cells. Bit lines are shared with multipleNAND strings. Typically, the bit line runs on top of the NAND strings ina direction perpendicular to the word lines and is connected to one ormore sense amplifiers.

Each memory cell can store data (analog or digital). When storing onebit of digital data, the range of possible threshold voltages of thememory cell is divided into two ranges which are assigned logical data“1” and “0.” In one example of a NAND type flash memory, the thresholdvoltage is negative after the memory cell is erased, and defined aslogic “1.” The threshold voltage after programming is positive anddefined as logic “0.” When the threshold voltage is negative and a readis attempted by applying 0 volts to the control gate, the memory cellwill turn on to indicate logic one is being stored. When the thresholdvoltage is positive and a read operation is attempted by applying 0volts to the control gate, the memory cell will not turn on, whichindicates that logic zero is stored.

In the case of storing multiple levels of data, the range of possiblethreshold voltages is divided into the number of levels of data. Forexample, if four levels of information is stored (two bits of data),there will be four threshold voltage ranges assigned to the data values“11”, “10”, “01”, and “00.” In one example of a NAND type memory, thethreshold voltage after an erase operation is negative and defined as“11”. Positive threshold voltages are used for the data states of “10”,“01”, and “00.” If eight levels of information (or states) are stored(e.g. for three bits of data), there will be eight threshold voltageranges assigned to the data values “000”, “001”, “010”, “011”, “100”,“101”, “110” and “111.”

The specific relationship between the data programmed into the memorycell and the threshold voltage levels of the cell depends upon the dataencoding scheme adopted for the cells. For example, U.S. Pat. No.6,222,762 and U.S. Patent Application Publication No. 2004/0255090, bothof which are incorporated herein by reference in their entirety,describe various data encoding schemes for multi-state flash memorycells. In one embodiment, data values are assigned to the thresholdvoltage ranges using a Gray code assignment so that if the thresholdvoltage of a floating gate erroneously shifts to its neighboringphysical state, only one bit will be affected. In some embodiments, thedata encoding scheme can be changed for different word lines, the dataencoding scheme can be changed over time, or the data bits for randomword lines may be inverted or otherwise randomized to reduce datapattern sensitivity and even wear on the memory cells.

Relevant examples of NAND type flash memories and their operation areprovided in the following U.S. patents/patent applications, all of whichare incorporated herein by reference: U.S. Pat. No. 5,570,315; U.S. Pat.No. 5,774,397; U.S. Pat. No. 6,046,935; U.S. Pat. No. 6,456,528; andU.S. Pat. Publication No. US2003/0002348. The discussion herein can alsoapply to other types of flash memory in addition to NAND as well asother types of non-volatile memory.

Other types of non-volatile storage devices, in addition to NAND flashmemory, can also be used. For example, a so called TANOS structure(consisting of a stacked layer of TaN—Al2O3-SiN—SiO2 on a siliconsubstrate), which is basically a memory cell using trapping of charge ina nitride layer (instead of a floating gate), can also be used with thepresent invention. Another type of memory cell useful in flash EEPROMsystems utilizes a non-conductive dielectric material in place of aconductive floating gate to store charge in a non-volatile manner. Sucha cell is described in an article by Chan et al., “A TrueSingle-Transistor Oxide-Nitride-Oxide EEPROM Device,” IEEE ElectronDevice Letters, Vol. EDL-8, No. 3, March 1987, pp. 93-95. A triple layerdielectric formed of silicon oxide, silicon nitride and silicon oxide(“ONO”) is sandwiched between a conductive control gate and a surface ofa semi-conductive substrate above the memory cell channel. The cell isprogrammed by injecting electrons from the cell channel into thenitride, where they are trapped and stored in a limited region. Thisstored charge then changes the threshold voltage of a portion of thechannel of the cell in a manner that is detectable. The memory cell iserased by injecting hot holes into the nitride. See also Nozaki et al.,“A 1-Mb EEPROM with MONOS Memory Cell for Semiconductor DiskApplication,” IEEE Journal of Solid-State Circuits, Vol. 26, No. 4,April 1991, pp. 497-501, which describes a similar memory cell in asplit-gate configuration where a doped polysilicon gate extends over aportion of the memory cell channel to form a separate select transistor.The foregoing two articles are incorporated herein by reference in theirentirety. The programming techniques mentioned in section 1.2 of“Nonvolatile Semiconductor Memory Technology,” edited by William D.Brown and Joe E. Brewer, IEEE Press, 1998, incorporated herein byreference, are also described in that section to be applicable todielectric charge-trapping devices. Other types of memory devices canalso be used.

FIG. 3 illustrates a non-volatile storage device 210 that may includeone or more memory die or chips 212. Memory die 212 includes an array(two-dimensional or three dimensional) of memory cells 200, controlcircuitry 220, and read/write circuits 230A and 230B. In one embodiment,access to the memory array 200 by the various peripheral circuits isimplemented in a symmetric fashion, on opposite sides of the array, sothat the densities of access lines and circuitry on each side arereduced by half. The read/write circuits 230A and 230B include multiplesense blocks 300 which allow a page of memory cells to be read orprogrammed in parallel. The memory array 100 is addressable by wordlines via row decoders 240A and 240B and by bit lines via columndecoders 242A and 242B. In a typical embodiment, a controller 244 isincluded in the same memory device 210 (e.g., a removable storage cardor package) as the one or more memory die 212. Commands and data aretransferred between the host and controller 244 via lines 232 andbetween the controller and the one or more memory die 212 via lines 234.One implementation can include multiple chips 212.

Control circuitry 220 cooperates with the read/write circuits 230A and230B to perform memory operations on the memory array 200. The controlcircuitry 220 includes a state machine 222, an on-chip address decoder224 and a power control module 226. The state machine 222 provideschip-level control of memory operations. The on-chip address decoder 224provides an address interface between that used by the host or a memorycontroller to the hardware address used by the decoders 240A, 240B,242A, and 242B. The power control module 226 controls the power andvoltages supplied to the word lines and bit lines during memoryoperations. In one embodiment, power control module 226 includes one ormore charge pumps that can create voltages larger than the supplyvoltage.

In one embodiment, one or any combination of control circuitry 220,power control circuit 226, decoder circuit 224, state machine circuit222, decoder circuit 242A, decoder circuit 242B, decoder circuit 240A,decoder circuit 240B, read/write circuits 230A, read/write circuits230B, and/or controller 244 can be referred to as one or more managingcircuits.

FIG. 4 depicts an exemplary structure of memory cell array 200. In oneembodiment, the array of memory cells is divided into M blocks of memorycells. As is common for flash EEPROM systems, the block is the unit oferase. That is, each block contains the minimum number of memory cellsthat are erased together. Each block is typically divided into a numberof pages. A page is a unit of programming. One or more pages of data aretypically stored in one row of memory cells. A page can store one ormore sectors. A sector includes user data and overhead data. Overheaddata typically includes an Error Correction Code (ECC) that has beencalculated from the user data of the sector. A portion of the controller(described below) calculates the ECC when data is being programmed intothe array, and also checks it when data is being read from the array.Alternatively, the ECCs and/or other overhead data are stored indifferent pages, or even different blocks, than the user data to whichthey pertain. A sector of user data is typically 512 bytes,corresponding to the size of a sector in magnetic disk drives. A largenumber of pages form a block, anywhere from 8 pages, for example, up to32, 64, 128 or more pages. Different sized blocks and arrangements canalso be used.

In another embodiment, the bit lines are divided into odd bit lines andeven bit lines. In an odd/even bit line architecture, memory cells alonga common word line and connected to the odd bit lines are programmed atone time, while memory cells along a common word line and connected toeven bit lines are programmed at another time.

FIG. 4 shows more details of block i of memory array 200. Block iincludes X+1 bit lines and X+1 NAND strings. Block i also includes 64data word lines (WL0-WL63), 2 dummy word lines (WL_d0 and WL_d1), adrain side select line (SGD) and a source side select line (SGS). Oneterminal of each NAND string is connected to a corresponding bit linevia a drain select gate (connected to select line SGD), and anotherterminal is connected to the source line via a source select gate(connected to select line SGS). Because there are sixty four data wordlines and two dummy word lines, each NAND string includes sixty fourdata memory cells and two dummy memory cells. In other embodiments, theNAND strings can have more or less than 64 data memory cells and twodummy memory cells. Data memory cells can store user or system data.Dummy memory cells are typically not used to store user or system data.Some embodiments do not include dummy memory cells.

FIG. 5 is a block diagram of an individual sense block 300 partitionedinto a core portion, referred to as a sense module 480, and a commonportion 490. In one embodiment, there will be a separate sense module480 for each bit line and one common portion 490 for a set of multiplesense modules 480. In one example, a sense block will include one commonportion 490 and eight sense modules 480. Each of the sense modules in agroup will communicate with the associated common portion via a data bus472. For further details, refer to U.S. Patent Application Publication2006/0140007, which is incorporated herein by reference in its entirety.

Sense module 480 comprises sense circuitry 470 that determines whether aconduction current in a connected bit line is above or below apredetermined threshold level. In some embodiments, sense module 480includes a circuit commonly referred to as a sense amplifier. Sensemodule 480 also includes a bit line latch 482 that is used to set avoltage condition on the connected bit line. For example, apredetermined state latched in bit line latch 482 will result in theconnected bit line being pulled to a state designating program inhibit(e.g., Vdd).

Common portion 490 comprises a processor 492, a set of data latches 494and an I/O Interface 496 coupled between the set of data latches 494 anddata bus 420. Processor 492 performs computations. For example, one ofits functions is to determine the data stored in the sensed memory celland store the determined data in the set of data latches. The set ofdata latches 494 is used to store data bits determined by processor 492during a read operation. It is also used to store data bits importedfrom the data bus 420 during a program operation. The imported data bitsrepresent write data meant to be programmed into the memory. I/Ointerface 496 provides an interface between data latches 494 and thedata bus 420.

During read or sensing, the operation of the system is under the controlof state machine 222 that controls the supply of different control gatevoltages to the addressed cell. As it steps through the variouspredefined control gate voltages corresponding to the various memorystates supported by the memory, the sense module 480 may trip at one ofthese voltages and an output will be provided from sense module 480 toprocessor 492 via bus 472. At that point, processor 492 determines theresultant memory state by consideration of the tripping event(s) of thesense module and the information about the applied control gate voltagefrom the state machine via input lines 493. It then computes a binaryencoding for the memory state and stores the resultant data bits intodata latches 494. In another embodiment of the core portion, bit linelatch 482 serves double duty, both as a latch for latching the output ofthe sense module 480 and also as a bit line latch as described above.

It is anticipated that some implementations will include multipleprocessors 492. In one embodiment, each processor 492 will include anoutput line (not depicted in FIG. 5) such that each of the output linesis wired-OR'd together. In some embodiments, the output lines areinverted prior to being connected to the wired-OR line. Thisconfiguration enables a quick determination during the programverification process of when the programming process has completedbecause the state machine receiving the wired-OR line can determine whenall bits being programmed have reached the desired level. For example,when each bit has reached its desired level, a logic zero for that bitwill be sent to the wired-OR line (or a data one is inverted). When allbits output a data 0 (or a data one inverted), then the state machineknows to terminate the programming process. In embodiments where eachprocessor communicates with eight sense modules, the state machine may(in some embodiments) need to read the wired-OR line eight times, orlogic is added to processor 492 to accumulate the results of theassociated bit lines such that the state machine need only read thewired-OR line one time.

During program or verify, the data to be programmed is stored in the setof data latches 494 from the data bus 420. The program operation, underthe control of the state machine, comprises a series of programmingvoltage pulses (with increasing magnitudes) applied to the control gatesof the addressed memory cells. Each programming pulse is followed by averify process to determine if the memory cell has been programmed tothe desired state. Processor 492 monitors the verified memory staterelative to the desired memory state. When the two are in agreement,processor 492 sets the bit line latch 482 so as to cause the bit line tobe pulled to a state designating program inhibit. This inhibits the cellcoupled to the bit line from further programming even if it is subjectedto programming pulses on its control gate. In other embodiments theprocessor initially loads the bit line latch 482 and the sense circuitrysets it to an inhibit value during the verify process.

Data latch stack 494 contains a stack of data latches corresponding tothe sense module. In one embodiment, there are 3-5 (or another number)data latches per sense module 480. In one embodiment, the latches areeach one bit. In some implementations (but not required), the datalatches are implemented as a shift register so that the parallel datastored therein is converted to serial data for data bus 420, and viceversa. In one preferred embodiment, all the data latches correspondingto the read/write block of m memory cells can be linked together to forma block shift register so that a block of data can be input or output byserial transfer. In particular, the bank of read/write modules isadapted so that each of its set of data latches will shift data in to orout of the data bus in sequence as if they are part of a shift registerfor the entire read/write block.

Additional information about the read operations and sense amplifierscan be found in (1) United States Patent Application Pub. No.2004/0057287, “Non-Volatile Memory And Method With Reduced Source LineBias Errors,” published on Mar. 25, 2004; (2) United States PatentApplication Pub No. 2004/0109357, “Non-Volatile Memory And Method withImproved Sensing,” published on Jun. 10, 2004; (3) U.S. PatentApplication Pub. No. 20050169082; (4) U.S. Patent Publication2006/0221692, titled “Compensating for Coupling During Read Operationsof Non-Volatile Memory,” Inventor Jian Chen, filed on Apr. 5, 2005; and(5) U.S. patent application Ser. No. 11/321,953, titled “Reference SenseAmplifier For Non-Volatile Memory, Inventors Siu Lung Chan andRaul-Adrian Cernea, filed on Dec. 28, 2005. All five of the immediatelyabove-listed patent documents are incorporated herein by reference intheir entirety.

At the end of a successful programming process (with verification), thethreshold voltages of the memory cells should be within one or moredistributions of threshold voltages for programmed memory cells orwithin a distribution of threshold voltages for erased memory cells, asappropriate. FIG. 6 illustrates example threshold voltage distributionscorresponding to data states for the memory cell array when each memorycell stores four bits of data. Other embodiment, however, may use moreor less than four bits of data per memory cell. FIG. 6 shows sixteenthreshold voltage distributions corresponding to data states 0-15. Inone embodiment, the threshold voltages in state 0 are negative and thethreshold voltages in the states 1-15 are positive.

Between each of the data states 0-15 are read reference voltages usedfor reading data from memory cells. For example, FIG. 6 shows readreference voltage Vr1 between data states 0 and 1, and Vr2 between datastates 1 and 2. By testing whether the threshold voltage of a givenmemory cell is above or below the respective read reference voltages,the system can determine what state the memory cell is in.

At or near the lower edge of each data state 0-15 are verify referencevoltages. For example, FIG. 6 shows Vv1 for state 1 and vv2 for state 2.When programming memory cells to a given state, the system will testwhether those memory cells have a threshold voltage greater than orequal to the verify reference voltage.

FIG. 7 illustrates that another embodiment of threshold voltagedistributions corresponding to data states 0-15 can partially overlapsince the ECC can handle a certain percentage of cells that are inerror.

Also note that the Vt axis may be offset from actual voltages applied tothe control gates as body effect through source or body biasing is usedto shift negative threshold voltage into the measurable positive range.Another point to note is that contrary to the equal spacing/width of thedepicted sixteen states, various states may have differentwidths/spacings in order to accommodate varying amounts ofsusceptibility to retention loss. In some embodiments, states 0 and/or15 are wider than the other states.

Each data state of FIG. 6 (or FIG. 7) corresponds to predeterminedvalues for the data bits stored in the memory cells programmed to therespective states. FIG. 8 is a table providing an example of the datavalues assigned to each data state 0-15. In one embodiment, a memorycell stores data in four different pages. The four pages are referred toas the 1^(st) lower page, 2^(nd) upper page, 3^(rd) higher page and4^(th) top page. FIG. 8 depicts the data in each page for each datastate 0-15. In one embodiment, each page is programmed separately. Inanother embodiment, all four data bits for a memory cell are programmedat the same time.

FIG. 9 is a table providing another example of the data values assignedto each data state 0-15. The data values of FIG. 8 utilize a Gray codeassignment so that only one bit changes between neighboring data states.This arrangement reduces the number of error bits if the thresholdvoltage of a memory cells is too low or too high.

FIG. 10 is a flow chart describing one embodiment for operatingnon-volatile memory cells. In many implementations, memory cells areerased (in blocks or other units) prior to programming. Memory cells areerased (see step 500), in one embodiment, by raising the p-well to anerase voltage (e.g., 20 volts) for a sufficient period of time andgrounding the word lines of a selected block while the source and bitlines are floating. Due to capacitive coupling, the unselected wordlines, bit lines, select lines, and source are also raised to asignificant fraction of the erase voltage. A strong electric field isthus applied to the tunnel oxide layers of selected memory cells and thedata of the selected memory cells are erased as electrons of thefloating gates are emitted to the substrate side, typically by aFowler-Nordheim tunneling mechanism. As electrons are transferred fromthe floating gate to the p-well region, the threshold voltage of aselected cell is lowered. Erasing can be performed on the entire memoryarray, a block, or another unit of cells. After the block of memorycells is erased, the various memory cells can be programmed as describedherein.

In step 502, soft programming is optionally performed to narrow thedistribution of erased threshold voltages for the erased memory cells.Some memory cells may be in a deeper erased condition than necessary asa result of the erase process. Soft programming includes applyingprogramming pulses to move the threshold voltage of the erased memorycells to a tighter threshold voltage distribution. Between eachprogramming pulse, a verify operation can be performed to determinewhether memory cells are within the proper erased threshold voltagedistribution. Memory cells that no longer need to be soft programmedprior to the soft programming process ending can be locked out offurther soft programming by raising the corresponding bit line. Theverify voltage can vary based on the particular implementation. Softprogramming is known in the art. More information about soft programmingcan be found in U.S. Pat. No. 6,661,711, which is incorporated herein byreference. Note that no one particular scheme for performing softprogramming is required. While soft programming may often be performedin association with an erase process, it may also be performed at thetime of programming.

In step 504, a request to program data is received. The request can bereceived from a host or other device. A dotted line is depicted betweensteps 502 and 504 to indicate that an unpredictable amount of time and,potentially, some intervening tasks (including other programmingoperations) may pass between steps 502 and 504.

In step 506, data is programmed into the memory cells by using amulti-pass programming process to program the memory cells from theerased threshold voltage distribution to any of the threshold voltagedistributions associated with valid data states. More details of step506 are provided below.

In step 508, the data memory cells are read and the data read isprovided to the user. For example, data from the memory cells isprovided to a host by the controller. A dotted line is depicted betweensteps 506 and 508 to indicate that an unpredictable amount of time and,potentially, some intervening tasks may pass between steps 506 and 508.In some cases, the data may never be read. The process of FIG. 10 isperformed by and at the direction of the controller 244 and/or controlcircuitry 220.

In general, during read and verify operations, the selected word line isconnected to a voltage, a level (e.g., Vr1 or Vv1) of which is specifiedfor each read and verify operation in order to determine whether athreshold voltage of the concerned memory cell has reached such level.The non-selected word lines are connected to an overdrive voltage(referred to as Vread) different from that of the selected word line.The overdrive voltage is typically chosen to be higher than the maximumthreshold voltage that can be programmed to a data memory cell. Thedifference between the overdrive voltage and the threshold voltage canalso be referred to as the overdrive. After applying the word linevoltage to the selected word line, the conduction current of the memorycell is measured to determine whether the memory cell turned on inresponse to the voltage applied to the word line. If the conductioncurrent is measured to be greater than a certain value, then it isassumed that the memory cell turned on and the voltage applied to theword line is greater than the threshold voltage of the memory cell. Ifthe conduction current is not measured to be greater than the certainvalue, then it is assumed that the memory cell did not turn on and thevoltage applied to the word line is not greater than the thresholdvoltage of the memory cell.

There are many ways to measure the conduction current of a memory cellduring a read or verify operation. In one example, the conductioncurrent of a memory cell is measured by the rate it discharges orcharges a dedicated capacitor in the sense amplifier. In anotherexample, the conduction current of the selected memory cell allows (orfails to allow) the NAND string that included the memory cell todischarge a voltage on the bit line. The voltage on the bit line ismeasured after a period of time to see whether it has been discharged ornot. More details about reading data can be found in US PatentPublication 2006/0221683, incorporated herein by reference in itsentirety.

FIGS. 11A-D describe a four pass programming process for programmingdata into memory cells that store four bits of data, as depicted in FIG.6 or FIG. 7. Prior to programming, all of the memory cells for a blockwill be erased into an erased threshold voltage distribution (see step500 of FIG. 10). For example, FIG. 11A shows a block of memory cells inan erased threshold voltage distribution E. In some embodiments, erasedthreshold voltage distribution E is below zero volts. In otherembodiments, erased threshold voltage distribution E is above zerovolts, or partially above zero volts. Negative threshold voltagedistributions can be shifted into the positive region by raising thesource voltage and thereby inducing a reverse body bias which tends toraise the threshold without changing the number of electrons on thefloating gate. This change in threshold voltage by application ofreverse body bias is due to the well known phenomenon of body effect. Inthe context most NAND memories the P-well constitutes the body. Tosimplify the diagrams, the Vt axes of FIGS. 11A to 11D are shifted withrespect to the actual VT axis so that all VT's appear to be positive. Inreality the negative VT's can be read with use of reverse body bias toraise these VT's above zero, and positive VT can be read without raisingthe body bias, in which case the maximum positive VT will be around 4Vand not as high as 6V or more as may be reflected by the shifted Vt axesof these figures.

The first programming pass is depicted in FIG. 11B. During the firstprogramming pulse, those memory cells that are to be eventuallyprogrammed to data states 0 through 7 will be programmed to a lowthreshold voltage distribution which is below the threshold voltagedistribution for state 0. For example, FIG. 11B shows those memory cellsbeing programmed to intermediate threshold voltage distribution LM01.The group of memory cells that are to be eventually programmed to datastates 8 through 15 will be programmed to an intermediate thresholdvoltage distribution labeled LM02 on FIG. 11B. In one embodiment,threshold voltage distribution LM02 is not greater than the thresholdvoltages for data state 7. Between programming pulses of this firstpass, two verify operations are performed: one for LM01 and one forLM02. Thus, between program pulses, two verify pulses will be used. Itmay be possible to only use one verify pulse (for LM01 only) at thebeginning of the program process until at least one memory cell hasreached LM01, at which time the system will use two verify pulses. Inother embodiments, the first programming pass can program to more thantwo threshold voltage distributions. In one embodiment, data is erasedto state 0 and memory cells that are to be eventually programmed to datastates 8 through 15 will be programmed to an intermediate thresholdvoltage distribution, while memory cells that are to be eventuallyprogrammed to data states 0 through 7 will remain in state 0. In anotherembodiment, LM01 can slightly overlap with the erased threshold voltagedistribution E since the ECC can handle a certain percentage of memorycells that are in error. Thus, the programming pass can end when all butK bits have verified, with K being a number that can be corrected byECC.

The second programming pass is depicted in FIG. 11C. Those memory cellsthat are to be programmed to data states 0 through 3 will be programmedfrom LM01 to threshold voltage distribution LM11. For example, FIG. 11Cshows threshold voltage distributions LM01 (dotted) and LM11 (solidline). Memory cells that are to be eventually programmed to data states3 through 7 will be programmed from LM01 to intermediate thresholdvoltage distribution LM12. Memory cells that are to eventually beprogrammed to data states 8 through 11 will be programmed from LM02 tointermediate threshold voltage distribution LM13. Those memory cellsthat are eventually to be programmed to data states 12 through 15 willbe programmed from LM02 to intermediate threshold voltage distributionLM14. During the second programming pass, up to four verify operationsneed to be performed between programming pulses: one verify operationfor LM11, one verify operation for LM12, one verify operation for LM13and one verify operation for LM14. Therefore, between each programmingpulse, there will be up to four verify pulses. It may be possible, insome embodiments, to reduce the number of verify pulses by using anintelligent scheme for determining when it is possible to omit some ofthe verify pulses because no memory cells could be near a particulartarget. Various schemes can be used. More information about efficientverification schemes can be found in U.S. Pat. No. 7,073,103,incorporated herein by reference in its entirety. Because the ECC canhandle a certain percentage of memory cells that are in error, inanother embodiment LM11 can overlap with LM12 and LM13 can overlap withLM14. Additionally, LM12 can overlap with LM01 or LM02, and LM14 canoverlap with LM02. Thus, the programming pass can end when all but Kbits have verified, with K being a number that can be corrected by ECC.

The third programming pass is depicted in FIG. 11D. Those memory cellsthat are eventually to be programmed to data state 0 and 1 will beprogrammed from LM11 to intermediate threshold voltage distributionLM21. Those memory cells that will be programmed to data states 2 and 3are programmed from LM11 to intermediate threshold voltage distributionLM22. Those memory cells that are to be programmed to data states 4 and5 are programmed from LM12 to intermediate threshold voltagedistribution LM23. Those memory cells that are to be programmed to datastates 6 and 7 are programmed from LM12 to intermediate thresholdvoltage distribution LM24. Those memory cells that are to be programmedto data states 8 and 9 are programmed from LM13 to intermediatethreshold voltage distribution LM25. Those data cells that are to beprogrammed to data states 10 and 11 are programmed from LM13 tointermediate threshold voltage distribution LM26. Those data cells thatare to be programmed to data states 12 and 13 are programmed from LM14to intermediate threshold voltage distribution LM27. Those memory cellsthat are to be programmed to data states 14 and 15 are programmed fromLM14 to intermediate threshold voltage distribution LM28. Because theECC can handle a certain percentage of memory cells that are in error,in another embodiment neighboring LM2 x distributions can overlap witheach other (e.g., LM26 can overlap with LM25). Additionally, LM2 xdistributions can also overlap with LM1 x distributions (e.g., LM26 canoverlap with LM13). Thus, the programming pass can end when all but Kbits have verified, with K being a number that can be corrected by ECC.

The fourth programming pass is depicted in FIG. 11E. In the fourthprogramming pass, memory cells to be programmed to data state 0 areprogrammed from LM21 to data state 0. Memory cells to be programmed todata state 1 are programmed from LM21 to data state 1. Memory cells tobe programmed to data state 2 are programmed from LM22 to data state 2.Memory cells to be programmed to data state 3 are programmed from LM22to data state 3. Memory cells that are to be programmed to data state 4are programmed from LM23 to data state 4. Memory cells that are to beprogrammed to data state 5 are programmed from LM23 to data state 5.Memory cells to be programmed to data state 6 are programmed from LM24to data state 6. Memory cells that are to be programmed to data state 7are programmed from LM24 to data state 7. Memory cells that are to beprogrammed to data state 8 are programmed from LM25 to data state 8.Memory cells that are to be programmed to data state 9 are programmedfrom LM25 to data state 9. Memory cells that are to be programmed todata state 10 are programmed from LM26 to data state 10. Memory cellsthat are to be programmed to data state 11 are programmed from LM26 todata state 11. Memory cells that are to be programmed to data state 12are programmed from LM27 to data state 12. Memory cells that are to beprogrammed to data state 13 are programmed from LM27 to data state 13.Memory cells that are to be programmed to data state 14 are programmedfrom LM28 to data state 14. Memory cells that are to be programmed todata state 15 are programmed from LM28 to data state 15. As discussedabove with respect to FIG. 7, another embodiment includes data states0-15 overlapping. Other embodiments may include intermediate LM statesthat overlap with one another, specially if the binary version of thedata remains until the entire block goes through the entire programmingsequence.

In one embodiment, when receiving a request to program data, the requestis accompanied by data for all four pages so that all of the memorycells on a word line can be programmed to any one of data states 0through 15. In that case, the four programming passes of FIGS. 11A-D areperformed based on the data received.

In another embodiment, a memory system can receive less than all fourpages at a time. In that case, the memory cells can be originallywritten as binary memory cells, with each page of data being written indifferent blocks. Memory will be erased to the erased state and one bitof data will be programmed into the memory cells so that some memorycells remain in the erased state (data 1) and some memory cells will bein the programmed state (data 0). When a first page of memory isreceived, that data is written to a first block of memory cells inbinary mode. When the second page of data is received, that data iswritten to a second block of memory cells. When the third page of datais received, that page of data is written to a third block of memorycells in binary form. When the fourth page of data is received, thatdata is written to a fourth block of memory cells in binary form. Onceall four blocks have been written, the system then can rewrite all fourblocks into one block by programming four bits of data per memory cellusing the four pass programming process of FIGS. 11A-D.

In another embodiment, one binary block is written page by page until itis completely written. Then another binary block is written page by pageuntil that is completely written too. Subsequently, a third binary blockand a forth binary block are written in the same manner as the first twoblocks. Once four complete binary blocks become available, then the datafrom these four blocks is condensed and written to block in the form of4 bits per memory cell. In one embodiment, the data from the binaryblocks is first sent to the controller chip where the binary ECCdecoding is performed, and then another type of ECC encoding appropriatefor 16 states per cell is performed in addition to data scrambling, andthe new data is sent back to the memory chip for the 4 state per cellblock to be written.

FIG. 12 is a flow chart describing a programming process for programmingmemory cells connected to a selected word line. Many differentprogramming techniques can be used with the present invention. In oneembodiment, the process of FIG. 10 is performed by and/or at thedirection of control circuitry 220 (state machine 222 provides thecontrol and power control 226 provides the appropriate signals) and/orat the direction of controller 244. In one embodiment, the process ofFIG. 12 is used to perform one pass of the four pass programming processof FIGS. 11A-E for memory cells connected (e.g. by their control gates)to one word line.

In step 624, a “data load” command is issued by controller 244 and inputto state machine 222. In step 626, address data designating the pageaddress is provided to the decoder circuitry. In step 628, a page ofprogram data for the addressed page(s) is input for programming. Forexample, 528 bytes of data could be input in one embodiment. That datais latched in the appropriate registers/latches for the selected bitlines. In some embodiments, the data is also latched in a secondregister for the selected bit lines to be used for verify operations. Instep 630, a “program” command is received from controller 244 andprovided to state machine 222. Triggered by the “program” command, thedata latched in step 628 will be programmed into the selected memorycells controlled by state machine 222 using a set of pulses applied tothe appropriate word line. In step 732, program counter PC maintained bystate machine 222 is initialized at 0.

In some embodiments, at least a subset of the programming passes willinclude identifying a program pulse associated with achieving aparticular result for a respective programming process and using theidentified program pulse to adjust programming for a subsequentprogramming process for the non-volatile storage elements. In oneexample, the system will identify the program pulse that results in thefirst memory cells reaching its target. In another example, the systemwill identify the program pulse that results the first N memory cellsreaching their target, with N being a number less than the amount oferrors that can be corrected by ECC. That identified pulse will have amagnitude. In one embodiment, the magnitude of the first pulse for thesubsequent programming pass will be set at the same magnitude as theidentified pulse. For example, if the fourth program pulse of the firstprogramming pass resulted in the first memory cell to reach its targetand the magnitude of that fourth program pulse is 14 volts, then themagnitude of the first program pulse of the second programming pass willbe set at 14 volts. In this way, observed behavior in one programmingpass can affect a subsequent programming pass.

In other embodiments, other variations can be performed. For example,the first pulse of the subsequent pass could be set based on theidentified pulse of the previous pass but not made to be exactly equalin magnitude.

Additionally, the program pulse from the previous programming pass canbe associated with a different event than the first memory cell(s)reaching its target. For example, a programming pulse can be identifiedwhen a memory cell reaches an intermediary level or other condition.

Thus, when starting a new programming pass, the system will access theinformation identifying the program pulse from the previous programmingpass that was associated with achieving the predefined particular result(step 634). The system will read the information identifying the pulsefrom a register, a flash memory cell or other non-volatile storagedevice. In one embodiment, step 634 is not performed for the first passof a multi-pass programming process and the initial magnitude of thefirst programming pulse is set at a nominal value of 10-17 volts,depending on device characterization.

In step 636, the magnitude of the first program pulse of the programmingsignal Vpgm is set based on the pulse identified from the previousprogramming pass and read in step 634. In step 640, one pulse of theprogram signal Vpgm is applied to the selected word line.

In step 642, the data states of the selected memory cells are verifiedusing the appropriate set of target levels. If it is detected that thethreshold voltage of a selected memory cell has reached the appropriatetarget level, then the memory cell is locked out of future programmingfor the remainder of the process of FIG. 10 by raising its bit linevoltage. Other lockout techniques can also be used.

If it is detected for the first time during this programming pass thatthe first N (where N could be 1 or more) memory cells being programmedhave reached their target data states, then in step 644 informationabout this program pulse is stored (step 646) in a non-volatile storagedevice such as a flash memory cell or register. This is the pulse thatwill be used to set the magnitude of the first pulse for the nextprogramming pass. In one alternative, step 644 is performed bydetermining whether N memory cells have been changed from not beinglocked out to being locked out of further programming because they havereached their target state. In some embodiments, the analysis of step644 would not be performed on memory cells that are programmed from LM01to LM11, from LM02 to LM13, from LM11 to LM21, from LM12 to 23, fromLM13 to LM25 and LM14 to LM27. In one embodiment, steps 644 and 646 arenot performed for the last pass of a multi-pass programming process.

After step 646, the process continues at step 648. If the current pulsewas not the first time during this programming pass that the first Nmemory cells being programmed have reached their target data states,then the process would continue at step 648 without performing step 646.

If all memory cells being programmed have reached their target datastates (step 648), then the programming process is complete andsuccessful. A status of “PASS” is reported in step 650. Note that insome implementations of step 648, it is checked whether at least apredetermined number of memory cells have been verified to have reachedtheir target states. This predetermined number can be less than thenumber of all memory cells, thereby allowing the programming process tostop before all memory cells have reached their appropriate verifylevels. The memory cells that are not successfully programmed can becorrected using error correction during the read process.

If, in step 648, it is determined that not all of the memory cells havereached their target states, then the programming process continues. Instep 660, the program counter PC is checked against a program limitvalue. One example of a program limit value is 20; however, other valuescan be used in various implementations. If the program counter PC is notless than the program limit value, then it is determined in step 666whether the number of memory cells that have not been successfullyprogrammed is equal to or less than a predetermined number. If thenumber of unsuccessfully programmed memory cells is equal to or lessthan the predetermined number, then the programming process is flaggedas passed and a status of PASS is reported in step 668. In many cases,the memory cells that are not successfully programmed can be correctedusing error correction during the read process. If, however, the numberof unsuccessfully programmed memory cells is greater than thepredetermined number, the program process is flagged as failed and astatus of FAIL is reported in step 670. If, in step 660, it isdetermined that the program counter PC is less than the program limitvalue, then the magnitude of the next Vpgm pulse is increased by thestep size and the program counter PC is incremented in step 662. Afterstep 662, the process loops back to step 640 to apply the next Vpgmpulse.

The program pulses of the programming signal Vpgm have rising magnitudesthat increase by a step size. In one embodiment, the step size isconstant so that every pulse of every programming pass increases fromits previous pulse by the same step size, In another embodiment, eachprogramming pass (or a subset of program passes) will utilize adifferent step size (see step 652). For example, the first programmingpass will have a step size of 0.4 volts; the second programming passwill have a step size of 0.7 volts; the third programming pass will havea step size of 0.35 volts, and the fourth programming pass will have astep size of 0.17 volts. Other values can also be used.

In an alternative embodiment, step 644 and step 646 will only beperformed during the first programming pass. Therefore, the threesubsequent programming passes will use a magnitude for a first programpulse be based on the first programming pass.

The process of FIG. 12 is performed once for each programming pass. In acase where there are 64 word lines and four programming passes per wordline, the process of FIG. 12 can be performed 4×64 times for each block.

FIG. 12 A shows an example of the programming voltage signal Vpgm havinga set of program pulses 680 that increase in magnitude by a step size.The step size is illustrated by the space between lines 684 and 686. Inbetween the program pulses are verify pulses (e.g., verify pulses 682).FIG. 12A shows three verify pulses that can be used to distinguish fourdata states. Only three pulses are depicted to make the figure easy toread. In one embodiment that uses sixteen data states, there could be upto fifteen verify pulses between program pulses. Looking back at FIG.12, step 640 includes applying one program pulse 680 and step 642includes applying one set of verify pulses (e.g., a set of up to fifteenpulses) 682.

FIG. 13 describes the order for programming the different programmingpasses on the different word lines of a block of memory. For example,FIG. 13 identifies the initial operation as the first pass (FIG. 11B)being performed on word line WL0, followed by the first pass beingperformed on word line WL1, followed by the second pass (FIG. 11C) beingperformed on WL0, followed by the first pass being performed on WL2,followed by the second pass being performed on WL1, followed by thethird pass (FIG. 11D) being performed on WL0, followed by the first passbeing performed on WL3, . . . followed by the fourth pass (FIG. 11E)being performed on WL63. The sequence of programming passes depicted byFIG. 13 is referred to as a diagonal sequence process because the orderproceeds diagonally along the chart of FIG. 13. In other embodiments,other sequences can be used.

In one embodiment, each pass can correspond to a page of data. Forexample, looking at FIGS. 8 and 11B, if the lower page is to receivedata 1 then the memory cell is programmed to LM01 and if the lower pageis to receive data 0 then the memory cell is programmed to LM02. Thus,after the first programming pass, it is possible to read whether thelower page data is 1 or 0 based on whether the threshold voltage of thememory cell is in LM01 or LM02. The second programming pass cancorrelate to the programming of the upper page of data. For example,looking at FIGS. 8 and 11C, if the second upper page of data is to bedata 1, then the memory cells would be programmed to either LM11 orLM13. If the upper page data is to be data 0, then the memory cells willbe programmed to LM12 or LM14. By testing which threshold voltagedistribution the memory cell's threshold voltage is in, the second pageof data as well as the first page of data can be read. The thirdprogramming pass can correspond to programming the higher page of data.For example, higher page data to be data 1 would be stored in LM21,LM23, LM25 and LM27. Data 0 would be stored in LM22, LM24, LM26 or LM28.The fourth pass can correspond to programming the top page of data intothe appropriate data state.

FIG. 14 is a flowchart describing a process for operating nonvolatilememory in an alternative embodiment that uses the results of softprogramming to adjust the programming passes. In step 700, a block ofmemory cells are erased. In step 702, soft programming is performed. Theprocess of soft programming could include performing the process of FIG.12 with some slight modifications. Step 634 would not need to beperformed and step 636 would always set the voltage for the initialpulse at some nominal value (e.g., between 10 and 17 volts depending ondevice characterization). When N memory cells have been locked out fromsoft programming because they have reached the appropriate targetthreshold voltage distribution for erased memory cells, theidentification of that programming pulse will be stored in step 646 ofFIG. 12. Information identifying the program pulse can be stored in aregister or a flash memory cell. In step 704, a request to program datais received. There is a dotted line between step 702 and 704 to indicatethat in a period of time and intervening steps can be performed in theinterim. In some embodiments, data can be programmed without receiving arequest to program data. In step 706, data is programmed into the memorycells from the erased threshold voltage distribution using themulti-pass programming process of FIG. 11A-E (or another multi-passprogramming process). In each of the programming passes of FIG. 14, themagnitude of the first program pulse is set based on the observedbehavior in the soft programming process of step 702. In step 708, datais read from the memory cells. In one embodiment, as depicted in FIG.14, soft programming performed in association with the erasing of memorycells. In other embodiments, soft programming can be performed inassociation with the programming (e.g., in response to a request toprogram).

FIG. 15 provides an example of a flowchart for performing step 706 ofFIG. 14. Steps 824-832 of FIG. 15 are the same as steps 624-632 of FIG.12. In step 834, the system reads the identification of the programpulse from the soft programming pass that was stored in step 702 of FIG.14. In step 836, the magnitude of the first program pulse for theprogramming signal Vpgm is set based on that identified pulse read instep 834. In one embodiment, the magnitude for the first program pulseof the current programming pass is set to be the same as the magnitudeof the pulse read in step 834. Thus, the information read in step 834could include magnitude information, pulse number information (fromwhich the magnitude can be calculated) or other information that can beused to identify the appropriate magnitude. In step 840, the programpulse is applied to the word line selected for programming. In step 842,the memory cells along the selected word line are verified. If all or apredetermined number of memory cells pass the verification (step 844),then the process is complete and successful in step 846 (status=pass).If not all or not enough of the memory cells verify, then it isdetermined in step 860 whether the program counter is below apredetermined limit. If it is below the predetermined limit, then atstep 862 the magnitude of the voltage of the program signal Vpgm isincreased by the step size and the program counter is also increased. Asdescribed above, the step size may vary based on which programming passis being performed. After step 862, the process loops back to step 840and the next program pulse is applied. If it is determined, in step 860,that the program counter is greater than the predetermined limit and thenumber of unsuccessfully programmed memory cells is less than apredetermined number (step 866), then the process completes successfullyin step 868 (status=pass). Otherwise, if the number of unsuccessfullyprogrammed memory cells is not less than or equal to the predeterminednumber (step 866), then the process fails in step 870.

The process of FIG. 15 is performed once for each programming pass. In acase where there are 64 word lines and four programming passes per wordline, the process of FIG. 15 can be performed 4×64 times for each block.

FIG. 16 provides a flowchart for another embodiment which uses the softprogramming process to determine the initial magnitude of theprogramming signal for a single pass programming process. In step 880, ablock of memory cells are erased. In step 882, soft programming isperformed and the system stores the identification of the program pulseassociated with when N memory cells verify to the erased thresholdvoltage distribution. As discussed above, N can be equal to 1, 2, . . ., depending on the device implementation. Step 882 of FIG. 16 is thesame as step 702 of FIG. 14. In step 884, a request to program data isreceived. In step 886, data is programmed into memory cells from theerased threshold voltage distribution to any of the data states using asingle pass programming process. In step 888, memory cells are read andthe data is provided to a user.

In one embodiment, step 886 is performed by implementing the process ofFIG. 15 once for each word line. That is, there is one pass for eachword line. In one embodiment, step 880 programs memory cells to theerased threshold voltage distribution E and step 886 programs the memorycells from erased threshold voltage distribution E to any of the datastates 0 through 15. In another embodiment, step 880 erases the memorycells to data state 0 and step 886 programs all or a subset of memorycells from data state 0 to any of data states 1 through 15.

FIG. 17 is a flowchart describing one embodiment of a process foroperating memory cells that uses the soft programming process to alterthe verification portion of the programming process. In step 902, memorycells of a block are erased. In step 904, a soft programming process isperformed. In one embodiment, memory cells can be erased to the erasedthreshold voltage distribution E. In other embodiments, the memory cellscan be erased to data state 0. During the soft programming, the systemwill identify the programming pulse to which the Nth memory cell (whereN equals 1 or higher) verified to the appropriate erased thresholdvoltage distribution. The system will also identify when all but Pmemory cells (where P equals 0 or higher) have verified. The values forN and P can be chosen based on ECC and/or device characterization. Byknowing the two pulses discussed above, the natural threshold voltagedistribution for the memory cells can be estimated. For example, if Nequals 2, P equals 8, and the step size is 0.4 volts, the naturalthreshold voltage distribution is (8−2)×0.4=2.4 volts. This informationallows for the determination of the number of verify operations requiredafter each program pulse. For example, the number of verify operationscan be reduced to (2.4+0.3)/0.35=8 verifies for each program pulse, eventhough there are sixteen data states. The first few and last few programpulses can have even less than eight verify operations. In the aboveequation, 0.3 represents the margining for program noise and 0.35represents the state-to-state threshold voltage separation for oneexample implementation.

In step 906, the two identified program pulses are stored in nonvolatilestorage (e.g., register, flash memory cell, etc.). In step 906, thesystem calculates an approximation of the threshold voltage distributionbased on the two pulses identified, as discussed above. In step 910, thesystem will calculate and store the maximum number of verify pulsesneeded based on the approximation of threshold voltage distribution andstep size, as discussed above. In step 912, a request to program data isreceived. The line between steps 910 and 912 is dotted to indicate thatan unpredictable amount of time and intervening operations can beperformed between step 910 and step 912. In step 914, data is programmedinto memory cells using a single pass programming process. In anotherembodiment, step 914 could include performing a multi-pass programmingprocess. In step 916, data is read from the memory cells and reported tothe user or host.

Step 914 can be implemented by using the process of FIG. 15; however,when performing a single pass programming process, the step size willnot be changed in each pass. Rather it will remain constant at a valuedetermined based on device characterization (e.g. 0.1-0.4 volts). In oneembodiment, the process of FIG. 17 could also include setting themagnitude of the initial program pulse based on soft programming, asdiscussed above. In one embodiment, word lines are programmed in anorder starting from the word line closest to the source line and workingtowards the bit line contact; for example, starting at WL0 and ending atWL63. Thus, the process of FIG. 15 would be performed 64 times, once foreach word line, in a single pass programming process.

FIG. 18 is one embodiment of a process for implementing step 842 of FIG.15 when the process of FIG. 15 is being used to implement step 914 ofFIG. 17. The process of FIG. 18 implements the concept that no more thana finite set of pulses will be used to verify, where that finite set is(probably) less than the full set. With respect to the example describedabove in regard to FIG. 17, no more than 8 verify pulses will be usedbetween program pulses rather than all 15 verify pulses. However, theset of 8 verify pulses that are applied changes over time based on thenatural threshold voltage distribution of the memory cells and thecurrent state of the programming process. Once the system knows thewidth of the natural threshold voltage distribution of the memory cells,it is estimates where that natural threshold voltage distribution isalong the graph of FIG. 6.

Step 910 of FIG. 17 calculated a maximum number of verify pulses. Thisdetermines the maximum width of the set of verify pulses. For example,if the maximum number of verify pulses is 8, then the set of verifypulses contemplated in FIG. 18 includes up to 8 verify pulses. As theprogramming process continues (through the various iterations of FIG.15), the set of verify pulses will move so that different verify pulsesbecome part of that set. For example, the set may start out onlyincluding a verify pulse for data state 0. Eventually the set will beaugmented so that it includes verify pulses for data state 0 throughdata state 7, followed by the set moving so that it includes verifypulses for data states 1 through 8, then data states 2 through 9, and soon.

In step 928 of FIG. 18, the set of verify is identified based on themaximum number of verify pulses and based on the highest data state tobe reached by any memory cell in the current programming process. InFIG. 18, the highest data state to be reached by any memory cell isreferred to as state X. The set of verify pulses will include the verifypulses to needed to verify the state that is one state higher than thehighest data state to be reached and verify pulses for the states belowthat state, but no more than the already determined maximum number ofverify pulses. In FIG. 18, the state that is one state higher than thehighest data state to be reached is referred to as state X+1. Forexample, if the programming process is just starting out, the highestdata state to be reached may be state E, so the set of verify pulseswould only include a verify pulse for state 0. If the memory cells haveonly reached state 5 and the already determined maximum number of verifypulses is 8, then the set of verify pulses would include verify pulsesfor states 0-6. If the programming process is significantly underway,memory cells have already reached state 7, and the already determinedmaximum number of verify pulses is 8, then the set of verify pulseswould include verify pulses for states 8-1. If the memory cells havealready reached state 10 and the already determined maximum number ofverify pulses is 8, then the set of verify pulses would include verifypulses for states 4-11. If the memory cells have already reached state13 and the already determined maximum number of verify pulses is 8, thenthe set of verify pulses would include verify pulses for states 7-14.

In step 930, the lowest verify pulse of the set of verify pulsesidentified in step 928 is applied to the selected word line. Themagnitude of that one pulse is set as the verify level for thecorresponding data state. In some embodiments, the verify level for adata state is equal to the lowest threshold voltage for that data state(see e.g., Vv1 for state 1, of FIG. 6). In step 932, the results of theprogram pulse are sensed. That is, it is determined whether the memorycell (or cells) of interest turned on or did not turn on. In step 934,the sensing result is compared to the data for each memory cell. If thememory cell is to be programmed to the target level just verified and ithas reached that level, then in step 936, the memory cell is locked outby raising its bit line voltage. In step 938, it is determined werethere anymore verify pulses within the set of verify pulses that havenot yet been applied. If not, then in step 942 it is determined whetherany of the memory cells have reached state X+1. If so, then theindication of the highest state (state X) is updated and the process ofFIG. 18 is complete (also completing step 842 at FIG. 15). If there aremore verify pulses for the set of verify pulses that have not yet beenapplied (step 938), then the next highest verify pulse is applied instep 940, and the process loops back to step 932. For example, when theset of verify pulses identified in step 928 includes verify pulses forstates 2-10, the loop of FIG. 18 (step 932-938) will be performed 8times.

After programming data, shifts in the apparent charge stored on afloating gate can occur because of coupling of an electric field basedon the charge stored in the adjacent floating gates. This floating gateto floating gate coupling phenomena is described in U.S. Pat. No.5,867,429, which is incorporated herein by reference in its entirety. Afloating gate that to a target floating gate may include neighboringfloating gates that are on the same bit line, neighboring floating gatesthat are on the same word line, or floating gates that are diagonal fromthe target floating gate because they are both on a neighboring bit lineand a neighboring word line.

The floating gate to floating gate coupling phenomena occurs mostpronouncedly between sets of adjacent memory cells that have beenprogrammed at different times. For example, a first memory cell isprogrammed at a level of charge on its floating gate that corresponds toone set of data. Subsequently, one or more adjacent memory cells areprogrammed at a level of charge on their floating gates that correspondto a second set of data. After one or more of the adjacent memory cellsare programmed, the charge level read from the first memory cell appearsto be different than the programmed charge level because the effect ofthe charge on the adjacent memory cell is being coupled to the firstmemory cell. The coupling from the adjacent memory cells can shift theapparent charge level being read a sufficient amount to lead to anerroneous reading of the data stored.

One attempt to solve the problem of floating gate coupling is found inU.S. patent application Ser. No. 11/377,972, “System For Performing ReadOperation On Non-Volatile Storage With Compensation For Coupling,” filedon Mar. 17, 2006, by Nima Mokhlesi, incorporated herein by reference inits entirety. The system described in that application included applyingcompensation voltages to neighboring memory cells while reading a targetmemory cell.

FIG. 19 describes a process for limiting the effect of floating gatecoupling when memory cells are programmed using the four passprogramming technique described herein. During a typical read process,the memory cell of a NAND string that is selected to be read receives avoltage at its control gate equal to (or approximately equal to) athreshold voltage that is in between two neighboring data states. Forexample, to determine whether a memory cell is in data state 0, thecontrol gate for the memory cell would receive Vr1 (see FIG. 6). Todetermine whether a memory cell is in state 1, two read operations wouldbe performed: a first read operation would apply Vr1 to the memory celland a second read operation would perform Vr2 to the memory cell (seeFIG. 6). Depending on whether the memory cell turned on or off inresponse to the control gate voltages, logic is used to determine whichstate the memory cell is in.

Similarly, during a verify process, a verify compare voltage is appliedto the control gate of the memory cell being verified. Typically thatverify compare voltage would be similar to the lowest threshold voltagefor the data state. For example, the verify voltage for the data state 1would be Vv1. The verify voltage for data state 2 would be Vv2. Whenperforming a read or verify process, the non-selected memory cells on anNAND string will receive a control gate voltage referred to as Vreadthat is significantly higher than the threshold voltages of data state15. This voltage Vread (the overdrive voltage) will make sure that allthe unselected memory cells will turn on and allow current to flow inthe NAND string so that the current flow of the NAND string will bedetermined only by the targeted memory cell.

The process of FIG. 19 provides a way of reading memory cells that havebeen programmed using the four pass programming process described abovein order to limit coupling from neighboring floating gates. The theoryapplied in the process of FIG. 18 is that during a read process thememory cell next to, and on the drain side of, the targeted memory cellon the same NAND string will sometimes receive a different voltage basedon its data in order to compensate for floating gate coupling. Forexample, when reading a memory cell connected to WL12, the memory cellconnected to WL13 will receive the different voltage. During the verifyprocess, all of the memory cells except for the source-side neighboringmemory cell will receive Vread, while the source-side neighboring memorycell will receive Vread less sum Δ (e.g., where Δ is 1 volt or anothervalue depending on device characterization). This other value will bedesignated as VreadX, where VreadX=Vread−Δ. During verify, all of thenon-selected memory cells will receive Vread except the neighboringmemory cell will receive VreadX. During a subsequent read process, ifthe source side neighboring memory cell is an even state (states 0, 2,4, 6, 8, 10, 12, 14), then the source side neighboring memory cell willreceive VreadX while all other non-selected memory cells will receiveVread. If, however, the source side neighbor memory cell is in an oddstate (state 1, 3, 5, 7, 9, 11, 13, 15), then the neighboring memorycell will receive Vread during the read process. This causes theneighboring memory cell to receive compensation if it is an odd state.

Looking back at FIG. 11E, memory cells in an odd state were more heavilyprogrammed from the respective intermediate state than memory cells inthe even states. Thus, neighboring memory cells in an odd state willhave greater coupling effect. If the four pass programming process isperformed using the sequence of FIG. 13, then the fourth programmingpass for a target memory cell was performed after the first threeprogramming passes for its neighbor but before the fourth programmingpass. Thus, only the fourth programming pass of the neighbor will causefloating gate to floating gate coupling on the target memory cell andbinary (on/off or low/high) compensation needs to be used depending uponwhether the fourth programming pass of the neighboring memory cell putthat memory cell in an odd or even state.

Step 980 of FIG. 19 includes receiving a read request for data on aparticular word line WLn. Thus WLn is the selected word line. In step982, a read process is performed for all of the memory cells on theneighboring word line WLn+1. Step 982 includes determining what datastate all of the memory cells connected to WLn+1 are in. For each bitline, an indication of whether the neighbor memory cell on WLn+1 is atan even state or an odd state is stored in step 984. This only requiresone bit (odd/even) of storage for each bit line. For example, a one bitlatch can be used. A read process is performed in step 986 for word lineWLn. During the process of step 986, Vread is applied to WLn+1. Step 986includes performing the compensation discussed above. In step 988, datafrom the read process of step 986 is stored for those bit linesassociated with a neighbor memory cell in the odd state. For example,the system can check the one bit latch to see if it indicates a neighborin an odd state and, if so, the results of step 986 are stored. Thosebit lines having a neighboring memory cell in the even state will notstore the date from the read process of step 986. In step 990, the readprocess is performed for word line WLn, with the neighbor word lineWLn+1 receiving VreadX. In step 992, data for those bit lines having aneighbor memory cell in the even state will store the data from the readprocess of step 990. In step 994, the stored data (step 992 or step 988)will be reported.

The foregoing detailed description of the invention has been presentedfor purposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed. Manymodifications and variations are possible in light of the aboveteaching. The described embodiments were chosen in order to best explainthe principles of the invention and its practical application, tothereby enable others skilled in the art to best utilize the inventionin various embodiments and with various modifications as are suited tothe particular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto.

1. A method for operating non-volatile storage, comprising: performing afirst programming process for a plurality of non-volatile storageelements; and performing a second programming process for saidnon-volatile storage elements subsequent to said first programmingprocess, said second programming process includes a set of verifyoperations that are at least partially based on said first programmingprocess.
 2. A method according to claim 1, wherein: said firstprogramming process is a soft programming process that is used totighten an erase threshold distribution.
 3. A method according to claim2, further comprising: determining an approximation of a thresholdvoltage distribution for said non-volatile storage elements based onsaid first programming process, said verify operations are at leastpartially based on said approximation of said threshold voltagedistribution.
 4. A method according to claim 1, wherein: said firstprogramming process applies programming pulses to said non-volatilestorage elements; said method further comprises identifying a firstprogram pulse needed to achieve a first result during said firstprogramming process and identifying a second program pulse needed toachieve a second result during said first programming process; and saidsecond programming process uses verify pulses between program pulses,said verify pulses are limited based on said first program pulse andsaid second program pulse.
 5. A method according to claim 1, furthercomprising: erasing said non-volatile storage elements, said firstprogramming process is a soft programming process performed inassociation with said erasing, said soft programming includes applyingpulses to said non-volatile storage elements; identifying a firstprogram pulse associated with a first set of one or more of saidnon-volatile storage elements completing said soft programming; andidentifying a second program pulse associated with all but a last set ofzero or more of said non-volatile storage elements completing said softprogramming, said second programming process includes performing verifyoperations based on said first programming pulse and said secondprogramming pulse.
 6. A method according to claim 1, further comprising:erasing said non-volatile storage elements, said first programmingprocess is a soft programming process performed in association with saiderasing, said soft programming includes applying pulses to saidnon-volatile storage elements; identifying a first program pulseassociated with a first set of one or more of said non-volatile storageelements completing said soft programming; and identifying a secondprogram pulse associated with all but a last set of zero or more of saidnon-volatile storage elements completing said soft programming; andidentifying a range based on said first programming pulse and saidsecond programming pulse, said second programming process includesapplying a number of verify pulses based on said range.
 7. A methodaccording to claim 1, further comprising: erasing said non-volatilestorage elements, said first programming process is a soft programmingprocess performed in association with said erasing, said softprogramming includes applying pulses to said non-volatile storageelements; identifying a first program pulse associated with a first setof one or more of said non-volatile storage elements completing saidsoft programming; and identifying a second program pulse associated withall but a last set of zero or more of said non-volatile storage elementscompleting said soft programming; and identifying a number of verifypulses based on said first programming pulse and said second programmingpulse, said second programming process includes applying up to saidnumber of verify pulses between program pulses in order to verifyprogramming.
 8. A method according to claim 7, wherein said applying upto said maximum number of verify pulses between program pulses in orderto verify programming comprises: (a) applying a pulse to control gatesof said non-volatile storage elements for a lowest state of a changingwindow of states; (b) sensing results; (c) locking out non-volatilestorage elements that have reached their respective targets; andrepeating steps (a)-(c) for other states in said window.
 9. A methodaccording to claim 7, wherein: said first set of one or more of saidnon-volatile storage elements consists of exactly one non-volatilestorage element; and said last set of one or more of said non-volatilestorage elements consists of zero non-volatile storage elements.
 10. Amethod according to claim 1, further comprising: erasing saidnon-volatile storage elements, said first programming process is a softprogramming process performed in association with said erasing, saidsoft programming includes applying pulses to said non-volatile storageelements; identifying a first program pulse associated with a first setof one or more of said non-volatile storage elements completing saidsoft programming; and identifying a second program pulse associated withall but a last set of zero or more of said non-volatile storage elementscompleting said soft programming; and calculating an approximation of athreshold distribution for said non-volatile storage elements based onsaid first program pulses and said second program pulse; and calculatinga maximum number of verify pulses based on said approximation of saidthreshold distribution and a program pulse step size for said secondprogramming process, said second programming process includes applyingup to said maximum number of verify pulses between program pulses inorder to verify programming.
 11. A method according to claim 10, furthercomprising: receiving a request to program data, said request isreceived after identifying said second programming pulse, said secondprogramming process is performed in response to said request.
 12. Amethod according to claim 1, further comprising: receiving a request toprogram data, said request is received after performing said firstprogramming process, said second programming process is performed inresponse to said request.
 13. A method according to claim 1, wherein:said first programming process situates said non-volatile storageelements in a state associated with being erased; and said secondprogramming process programs user data into said non-volatile storageelements.
 14. A method according to claim 1, further comprising:performing other programming on other non-volatile storage elementsafter said first programming process and prior to said secondprogramming process.
 15. A method according to claim 1, wherein: saidnon-volatile storage elements are multi-state NAND flash memory devices.16. A method for operating non-volatile storage, comprising: performinga first programming process for a plurality of non-volatile storageelements, said first programming process includes applying programmingpulses to said non-volatile storage elements; identifying a firstprogram pulse associated with a first set of one or more of saidnon-volatile storage elements completing said first programming process;and identifying a second program pulse associated with all but a lastset of zero or more of said non-volatile storage elements completingsaid first programming process; and performing a second programmingprocess for said non-volatile storage elements subsequent to said firstprogramming process, said second programming process uses verify pulsesbetween program pulses, said verify pulses are limited based on saidfirst program pulse and said second program pulse.
 17. A methodaccording to claim 16, wherein said second programming process usesverify pulses as part of a verification process, said verificationprocess comprises: (a) applying a verify pulse to control gates of saidnon-volatile storage elements for a lowest state of a changing window ofstates; (b) sensing results; (c) locking out non-volatile storageelements that have reached their respective targets; and repeating steps(a)-(c) for other states in said window.
 18. A method according to claim16, further comprising: erasing said non-volatile storage elements, saidfirst programming process is a soft programming process that isperformed in conjunction with said erasing.
 19. A method according toclaim 16, further comprising: calculating an approximation of athreshold distribution for said non-volatile storage elements based onsaid first program pulse and said second program pulse, said verifypulses are limited by said approximation of said threshold distribution.20. A method according to claim 16, further comprising: calculating anapproximation of a threshold distribution for said non-volatile storageelements based on said first number and said second number; andcalculating a maximum number of verify pulses based on saidapproximation of said threshold distribution and a program pulse stepsize for said second programming process, said second programmingprocess includes applying up to said maximum number of verify pulsesbetween program pulses in order to verify programming.
 21. A method foroperating non-volatile storage, comprising: erasing a plurality ofnon-volatile storage elements; soft programming said plurality ofnon-volatile storage elements in association with said erasing;identifying a first program pulse associated with a first set of one ormore of said non-volatile storage elements completing said softprogramming; and identifying a second program pulse associated with allbut a last set of zero or more of said non-volatile storage elementscompleting said soft programming; and programming said non-volatilestorage elements, said programming includes perform verify operationsbased on said first program pulses and said second program pulses.
 22. Amethod according to claim 21, further comprising: identifying a rangebased on said first program pulses and said second program pulse, saidprogramming includes applying additional program pulses and verifyingbetween said additional program pulses, said verifying is limited bysaid range.
 23. A method according to claim 21, further comprising:identifying a maximum number of pulses based on said first programpulses and said second program pulse, said programming includes applyingadditional program pulses and verifying between said additional programpulses, said verifying includes applying verify pulses up to saidmaximum number of pulses.
 24. A method according to claim 21, furthercomprising: calculating an approximation of a threshold distribution forsaid non-volatile storage elements based on said first program pulsesand said second program pulse; and calculating a maximum number ofverify pulses based on said approximation of said threshold distributionand a program pulse step size for said programming process, saidprogramming includes applying additional program pulses and verifyingbetween said additional program pulses, said verifying includes applyingverify pulses up to said maximum number of pulses.